Band pass filters



`lune 5, 1956 M. DlsHAL. 2,749,523

BAND PASS FILTERS Filed Dec. 1; 1951 2 sheets-sheet 1 ATTENUAT/ON l I IUUTPl/T INVENTOR M/L TON /5H/4L June Y5, 1956 M. DlsHAL BAND PASSFILTERS 2 Sheets-Sheet 2 Filed Dec. l 1951 INVENTOR MILTON DSHALATTORNEY Unite 19 Claims. (Cl. 333-73) This invention relates tofilters, and more particularly `tcYV'I-IF, UHF, andmicrowave band passfilters of the cavity resonatortype having high rates of cnt-olf.

It 1'has been understood by'those skilled in the art for Amany yearsthat filter networks whose transfer impedance or admittance have bothpoles and zeros can be designed to producehigher rates of cut-ofi thanis possiblewith Acomparable networks having only poles and no zeros. Forinstance, 11i-derived .filter configurations are ,capable of `greaterrates of cut-off than comparable constant-k configurations. Whenoperating in the VHF, UHF, Vand microwave region, it has been furtherVunderstood that band passfilters are usually required to supply'srnallpercentage 'pass bands, vand to successfully lmeet this requirement, theratios between the reactances, as meas- 'ured at the mid frequency, ofall the various elements in the 'configuration must conform to rigidtolerances. An equivalent concept is embodied in the statement Vthat'theresonant frequency of each resonator and the coefficients of couplingbetween resonators must .conform to rigid tolerances.

Heretofore, *it has :been `attempted to provide VHF, UHF, and microwaveband pass filters having a high rate of'cut-oif due to zeros in thetransfer characteristic by utilizingthe old .wellknown configurationsand' concepts of standard filter theory. Thishas resulted incircuitshaving both a configuration and required reactance ratios, `that are notpracticably obtained.

It `is `one of the objectives lof the present invention to *provide :aband -pass filter having both a Vconfiguration `and the requiredreactanee ratios which are practicable to obtain and having provisionsto vary the resonant "frequency of :such a configuration 'to accommodatea v-rangeof frequencies in the VI-EF, UHR'andtmicrowave v region. Byernp`loying coaxial orl cavity resonatorswhich -are fanalogous -to I a tlow frequency resonant 'circuit with :eah'resonator correctly Ycoupledtothe `following resonator =eitherfma`gnetieally 'or electrically, Iprovide'al'band pass Efilter-operable in Vvthis VHF, UHFfand microwave7region. 7The-'dimensions `of the cavity resonator areso '-s'elected-thatitw'ill resonate 'at Va desired frequency, or `means maybeprovided'to produceresonanceatthe'desired'frequency 'The dimensions ofthe couplingfopeny"ings 'between adjacent resonators producesVthe'zcolrrect 'eoe'iiicient of coupling between adjacent resonators.'Therefore,byfproperly adjusting the =resonators resonant `frequencyandfthe coefficients of coupling 'between' adjatserttiresonators `andlthe coupling of the generator :land loadtovthe firstandlast'resouators, respectively,a Vtrans- .ferzcharacteristi'c .isIobtained having complex frequency :poles '.Another object o'f thisinventionis'to'provide aifilter lsystemfliavin g iboth poles and zerosTin 'the transfer Schar- .,aeteristictotproducea-:highrate'of entr-off.VThis-object iis t obtained by s carrying :the `aforenlentionedLfilterinetwork nonesstep :further to :introduce vzeros randtherebytprovi'de Leadiigh nate of1;cutolf.similar;tothatzobtainedwithnnmderived filter configuration. These zeros orinnitefat- States Patent 2,749,523 Patented June 5, 1956 ICC '2tenuation points areprovided by two coupling lines -between alternatecavities of a filter configuration. Thus, inaddition to the regularcoupling connection between adjacent resonators, alternate reasonatorsto which coupling lines are connected have a pair of probetype'couplings for `each'pair of coupling lines which .connect to them.Due to the fact that it is -possible to get oppositely phased inductiveprobes, there are fourdfferent cornbinations that can be used to make upthe above mentioned two pairs of probes and their coupling lines. Therequired electrical length ofthe V4coupling lines between alternateresonators will depend upon the probe .combinations employed in `each of`the alternate resonators. In general, including'the end effects of theprobes, the coupling lines ywill be some multiple of avquarterwvavevlength long. The basic requirement for the probecombinations and their'associated coupling lines is'that, 'one lineshall produce between the two resonators a knet capacitive type couplingwhich is essentially constant over the frequency band for which thefilter is to be used, and the other line shallproduce between the tworesonators a net'constant inductive type coupling `and `the .two typesof coupling shall be essentially equal in magnitude. The plurality ofcavity resonators may be any desired number with the restrictionthat'the aforementioned coupling lines must couple alternate cavities inthe 'manner indicated and no coupling lines may by-pass energy pastothercoupling lines. An explanation of Athe production of zeros is thatat some frequencies there is a 180 phase Idierence between the 'netcurrent injected into 'the of this invention and the manner of attainingthem will be best understood by referenceto the following descriptionof'enibodiments of the invention taken in conjunction with theaccompanying drawings, wherein:

Fig. 1 is a longitudinal cross-section view of a band pass v'filteroperable inthe VHF, UHF, and microwave regions inaccordance with theprinciples of this invention;

`Fig. y2 is a cross-Section view `partiy in elevation 'along line 2 2onFigjl;

Fig. 3 shows l'the vresponse curves 'for the Vband pass filter,onewith'and'the other without zeros; and

VFigs.'4, A'5, 6, and 7 are longitudinal cross-'Section views 'ofband-passfilterseach `showing a different embodiment of the iinvention.

'Referring to Figs. 1 and 2 of the drawing, a band pass -filtercapable-of 'operating in the VHF, UHF,and.micro -Wave Vregions isshownas-comprising five coaxial resonators 1, 2, 3 4, and 5 having tuningslugs 6, 7,'8, 9.and 'l0 ther-ein. Cavity resonator ll'v/ith its tuningslug 6 will lelectric .'field present therebetween.

`be described inA detail inrelation `to its configuration whichwilltsuflice as 'adescription for cavity resonators 2,v 3, 4, land v5since all resonators herein employed are similar.

4Resonator 1 is considered coaxial due to the inclusion offtun'inglslug'6in the cavity. Tuning slug 6 behaves similar Zto thecenter `conductorof a coaxial line while the 'walls of'resonatorl behave as the outerconductor pendicular-'relationwith the lines of force of the electricifield. .To enable Vthe :establishment of fields in such arelaltion, itisinecessary :to have 'the dimensions of the cavitycorrectiforzafdesired frequency. vFor example, a filter operating at..1'5001rnc. could employ l1/2 cubes, the

walls being approximately 1%16" thick and the slugs should beapproximately B/s in diameter. The dimensions of the cube are importantas those skilled in the art will realize, while the thickness of thewalls and diameter of the slugs are variables that will depend on theskineffect requirement and desired amount of coupling from the resonatorat the operating frequency. Tuning slug 6 is adjustable for frequencyselection and aperture 11, which mates with aperture 12, sets thecoeiicient of coupling to the adjacent resonator, the approximate designequations for coefficient of coupling between adjacent resonators beingB W3 ab fo Coaxial resonators 1, 2, 3, 4, and 5 may be held together bysoldering, bolt and nuts, or screws, or by some other suitable means. ifdesired, the walls of the several cavities may be made integral.Coupling between adjacent coaxial resonators 1, 2, 3, 4, and 5 isaccomplished by apertures, such as apertures 1I. and 12 in the walls ofcoaxial resonators 1 and 2. The arrangement shown in Fig. l of thedrawing is such that there is magnetic and electric coupling progressingalternately from resonator 1 to resonator 5 of the filter. Thisarrangement of coupling by alternate fields reduces the directfeedthrough of frequency, on say the magnetic field, which would tend toreduce the skirt selectivity of the filter. in other words, byalternating the adjacent resonator coupling between magnetic andelectric fields, it is necessary for the energy in the magnetic field tobe transferred to the electric field by a resonator before it can passon to the next resonator so that the energy must pass selectively fromone cavity to another giving an overall optimum selectivity. Using thesame line of reasoning, it can be seen that the input aperture 13 andthe output aperture 14 should follow the same pattern. It is conceivablethat the location of apertures 13 and 14 could be such as not to followthis pattern with the only result being that far out on the skirts ofthe response the attenuation may not be as great as possible.

The location of the strongest electric field is in the region 1S nearthe end of tuning slug 6 and the strongest magnetic field is in theregion 16 far from the end of tuning slug 6. To obtain maximum resultsfrom the filter, it is helpful to locate the adjacent couplingapertures, such as 11 and 12, and apertures 13 and 14 in these regionsof maximum field strength. However, the performance of the filter is notdependent upon locating the apertures in the region of the strongestfields as long as coupling means are located adjacent to the fields.Apertures 11 and 12 when located adjacent the magnetic field may be ovalor rectangular in shape with the longer dimension in parallel relationwith magnetic lines of force, and when such apertures are locatedadjacent the electric field, they may be circular in shape althoughother shapes may be employed.

A filter as described to this point is one which contains poles, but nozeros, much like a constant-k configuration in low frequency work. Theresponse curve of such a VHF, UHF, and microwave band pass filter willbe seen in Fig. 3 curve A. It will be noticed that this arrangementalone is not as good as may be desired since the rate of cut-off asshown by the slope may not be as sharp as required. Coupling separatelyalternate resonators as shown in Fig. l accomplishes this object of myinvention, resulting in curve B, Fig. 3. A coupling line 17 with its endprobes 21 and 21a is used to produce a resultant inductive couplingbetween resonators 1 and 3, while a coupling line 18 with its end probes22 and 22a ISvndiaeent p produces capacitive coupling between resonators1 and 3.

coupling lines 17, 18 and 19, 20 will depend upon probe L combinationstherein, but in general will be some multiple of a quarter wavelengthlong.

One of the four different combinations of coupling probes and linelengths is shown in resonators 1 and 3 of Fig. 1 wherein the magneticprobes 21 and 21a, connected to the one-quarter wavelength long couplingline 17, and the electric probes 22 and 22a, connected to theone-quarter wavelength long coupling line 18, are in such a physicalarrangement that each current injected into resonator 3, due to thevoltage across resonator 1 is shifted and -{9G, respectively, withrespect to the voltage across resonator 1. This is one of thefundamental requirements that must be satisfied by a pair of couplinglines and their end probes. Another fundamental requirement is that oneof the aforementioned 90 currents increases in magnitude in directproportion to frequency, and the other 90 current decreases in magnitudein direct proportion to frequency.

There is in addition a fundamental requirement upon the adjacent orintermediate resonator couplings which may be stated as follows: Thephase relation between the net current injected into resonator 3 by thealternate resonator coupling and the net current injected into resonator3 by the intermediate resonator coupling must be such that at thedesired frequency of infinite attenuation they are opposing in phase.Thus, at these frequencies where the phase of the waves are out ofphase, by proper adjustment of the amount of the adjacent andintermediate coupling, it is possible to completely cancel both themagnetic and electric propagation, thereby producing zeros or insertinginfinite attenuation points, such as 23 on curve B, Fig. 3. Thepreceding equation gave the approximate required value for the adjacentcouplings in order to produce a desired pass band width, and the belowequation `gives the approximate required value for the alternatecouplings The second infinite attenuation points 24 on curve B areproduced in a similar manner, by employing coupling lines 19 and 2f)cooperating with coaxial resonators 3, 4 and S wherein the various phaserelations are identical. Through the frequency band which the filter isoperative, the capacitive type coupling coefficient of coupling line 18and the inductive type coupling coemcient of coupling line 17 areessentially constant and equal in magnitude.

Fig. 4 in the drawing illustrates another embodiment of my inventionwherein the individual cavity resonators are identical with the coaxialresonators 1, 2, 3, 4, and 5 discussed above. The difference between theembodiments is the arrangement whereby the adjacent coupling apertures25, 26, 27, 28 are situated alternately in an electric field region anda magnetic field region, with both the input aperture 29 and outputaperture 30 of the filter being situated adjacent a magnetic fieldregion. The results obtained with this arrangement are substantiallyidentical with that of the aforementioned arrangement with possibly theexception that the attenuation far from the pass band is slightlydecreased due to the adjacent coupling aperture 28 and the outputaperture 30 being in the region of the same energyfield for reasonspreviously mentioned. This apparent disadvantage of the couplingarrangement in coaxial resonator 31 may be readily overcome by shiftingaperture 30 vertically in order that it may couple from the electricfield rather than the magnetic field thus reducing the possibility ofdirect feedthrough of unwanted frequency on the magnetic field. Voltagein coupling lines 32, 33 and 34, 35 in cooperation with the voltagepassing through resonators 2 and 4, respectively, produces the points ofinfinite attenuation as discussed with reference to Figs. 1 and-2 of thedrawing.

:arr-49ans .The combinations of. magnetic probes '.36 fandi36a .and:electric probes :.37 .and 37 a with ftheinrespective .coupling 4lines Y33 Vand 62, :are .physically arranged Vto satisfy the phaserequirements. as set `Vforth -in t the embodiment shown in diig. lfbyhaving-.theflength ofv coupling :lineiSZHthesame as the length ofcouplingtlineS 'Ewith ,the Ytot-al length :of reachline being2some4odd.tmultipleY-.of .a quarter iwave- -.length.

v;Still another .fembodiment of my .invention .is .shown :in Pig. 5Y of.the drawing. This `variation-operates Von the -..same principle :as4the `previous embodiment as -to the .development of zeros. .The cavityresonators-arerectan -ygular .waveguides.38 39, 40, 41, and 42-which:operate tin the TEon'mode. ,Physical dimensions of .theindividualwaveguide resonator-s are approximately 0707A along, '10T/.07x wide,-where )t :is the .free:spacewavelength, and .theY height .being .chosento -satisfy the requirement .of electric fieldstrength. .As is .knownvthe-electric .field fis concentrated .betweenpointsAS ,and 44 .in ia.typical fwave- .guide :resonator tof this type :while .the.magneticfield .ris l '..circulardn cross section, concentric withtheconcentrated electricv eld.

.In the arrangement indicated, 'the tinput tothe filter -is by. means of.loop 45 .through aperture"46 in the wall adjacent to themagnetic fieldof waveguide `resonator 38. `The voltage'oscillates betweenthe magnetic:eldf'and the electric `field -with part of 4the electric .field being.coupled to waveguide-resonator 39 through-circular .aperture-44. .A.resulting :electric .and magnetic field oscillation results inwaveguide resonator 39 and part of the magneticfleld is coupled towaveguide resonator 40 via a rectangular slit aperture ltS-throughthewalltherein. This -alternating process from electric to :magneticcoupling continues progressively through the filter until .the selected.energy is coupled out ofthe system through aperture .-49 .in'fthe v.electric field of waveguide resonator'42. The zerosl in '.thetransfercharacteristic are produced-as heretofore .ex- Epllained inconnection with Fig. '1 by employingcoupling Llines '50, 51,152, and`53and the associated probes. With the probes as shown, lines `50 and'51must be an identical odd number of quarter Wavelengthsvlong Lines 52 and53 .must also be ,an identical `odd number fof quarter` wavelengthslOng.

A further embodiment of my inventionis shown in Fig. 6 which illustratesthree adjacent waveguideresonators'SA, 55, and 56, the dimensions ofwhich are discussed A.in V'coninection Vwith.=Fig. A5, with adjacentcoupling .provided Yby quarter wavelength coaxial lines 58 and 59. v

Production of zeros is accomplished by employing the third of the fourcoupling probe combinations hereinbefore mentioned. As shown because ofthe opposite sense of the coupling loops 64 and 64a, the coupling lines62 and 63, with their associated probes 64 and 64a and 65 and 65a, mustdiffer in length by a half wavelength. The extra half wavelength may beplaced in the coupling line 62 connecting the magnetic probes. Thisarrangement then establishes, in accord with the fundamental phaserequirements outlined in the discussion of Fig. 1, the requirement thatcoupling loops 61 and 60 be oppositely bent as shown.

A fourth coupling combination may be achieved by placing the requiredextra half wavelength in the coupling line 63 connecting the capacitiveprobes. This will require one coupling loop 60 or 61 to be reversed fromthe position shown to establish the proper phase relation as outlined inthe discussion of Fig. 1 which allows achievement of the desired filterhaving a high rate of cut-off.

Fig. 7 illustrates another embodiment wherein waveguide resonators 64,65, and 66 are adjacently coupled by quarter wavelength waveguides 67and 68. Alternate resonator coupling is provided through coupling lines69 and 70 in cooperation with magnetic coupling probes 71 and 71a andcapacitive coupling probes 72 and 72a. The physical arrangements ofthese magnetic probes 71 and 71a are in the opposite sense withreference to their slt) V.respectivecapacitivetzcouplinggprobes 72 and72a, therefore, the length of the coupling linesf69 and i70=rnust differ.rinflengthfbyone-half -wavelangthttoproperly attain theA.aforementioned filter results. Because vadjacent coupling IwaveguidesV67 and 68 are bothof the positive -mutual inductance type, it isnecessary to add the required extra half wavelength in athecouplingfline70 joining the capaci- Y.tive probes 72 ,and .72a.in order that therequired 'filter `energy. intothe .series-of resonators, output terminal.means to-remove energyfromtsaidtseries of. resonators, saidseries ofresonators .including two ,groups, one `of said groups .consistingfof.the .odd ones of .saidseries of resonators and :the other of saidgroupsconsisting'of ,the 'even ones of isaid-vscriesof resonators, and.apferiodic means coupling ,themember-s of oneof.said,,groups in tandem.

{2.A band'passfilter according toclaim 1, wherein saidlmeanscouplingsaid resonators in series includes walls of .saidresonators having-.apertures therein, certain of .said .apertures being.configured .anddisposedfor coupling with `.thevelectric -field.andfothervofasaid apertures being con- :figuredV and disposed .for4.coupling with the .magnetic field of. .said resonators.

3. A band pass lfilter .accordingto Vclaim 2 lwherein .saidaapertures rcoupled with lthe electric field are circular .andsaidapertures coupledwiththe magnetic .field are .elongated inta directionlparallel to ,themagnetic lines of ...force 4. IA band pass filter according vto .claim.2, wherein the energy inreach `of? said .resonators :is coupled between.the electriciand magnetic .fields-and :said .means coupling.saidresonators series couples ithe energyV progressively .betweenvthelelectric and:magnetc.fields of said series of resonators.

5. `A band,pass.filteraccording-to claim 2, wherein said Lmeanscouplingsaid y.resonators in series .includes coaxial .transmission linesxhavingallengthequal to=odd multiples .of one-quartenwavelength.

s .6. ,A.bandp.ass filter .according-.totclaiml lwherein said meanscoupling said resonators in series includes waveguide couplers having alength equal to odd multiples of one-quarter wavelength.

7. A band pass filter according to claim l, wherein said input terminalmeans is disposed to be coupled with the electric field of the first ofsaid series of resonators and said output terminal means is disposed tobe coupled with the magnetic field of the last of said series ofresonators.

8. A band pass filter according to claim 7, wherein said input terminalmeans comprises a probe extending into the first of said series ofresonators for capacitive coupling to the electric field therein whilesaid output terminal means comprises a loop extending into the last ofsaid series of resonators for inductive coupling to the magnetic fieldtherein.

9. A band pass lter according to claim 7, wherein said input terminalmeans comprises a loop extending into the first of said series ofresonators for inductive coupling to the magnetic field therein whilesaid output terminal means comprises a probe extending into the last ofsaid series of resonators for capacitive coupling to the electric iieldtherein.

l0. A band pass filter according to claim l, wherein said aperiodicmeans includes a pair of coupling lines, one of said lines havingcapacitive probes and the other of said lines having inductive loops forcapacitive and inductive coupling of energy between the members of saidone of said groups.

11. A band pass filter according to claim 10, wherein said couplinglines include coaxial coupling lines having a length equal to oddmultiples of one-quarter Wavelength long.

12. A band pass filter according to claim 1, wherein said resonators arecoaxial resonators including a cylindrical tuning slug.

13. A band pass filter according to claim 1, wherein said resonators arerectangular waveguide resonators.

14. A band pass filter comprising five coaxial resonators each includinga cylindrical tuning slug, means coupling said resonators in series,input terminal means to introduce signal to said series of resonators,output terminal means to remove signal from said series of resonators,said series of resonators including two groups, one of said groupsconsisting of the odd ones of said series of resonators and the other ofsaid groups consisting of the even ones of said series of resonators,and aperiodic means coupling the members of one of said groups intandem.

15. A band pass filter according to claim 14, wherein each of saidresonators encompassa magnetic field and electric field in couplingrelation with each other and said means coupling said resonators inseries are so disposed with respect to the magnetic and electric fieldsof said resonators that the energy traverses said series of resonatorsalternately on the electric and magnetic fields of said resonators, saidinput terminal means being disposed in coupling relation with theelectric field of the first of said series of resonators and said outputterminal means being disposed in coupling relation with the magneticfield of the last of said series of resonators.

16. A band pass filter according to claim 15, wherein said aperiodicmeans comprises two pairs of coaxial coupling lines having a lengthequal to an odd multiple of one-quarter wavelength, one pair of saidcoupling lines coupling the first and the third resonators while theother pair of said coupling lines couple the third and fth resonators,one coupling line of each pair of coupling lines having capacitivecoupling probes and the other coupling line of each pair of couplinglines having inductive coupling probes for magnetic and electric fieldenergy coupling between the members of said one of said groups.

17. A band pass filter comprising five rectangular waveguide resonators,means coupling said resonators in series, input terminal means tointroduce signal to said series of resonators, output terminal means toremove signal from said series of resonators, said series of resonatorsincluding two groups, one of said groups consisting of the odd ones ofsaid series of resonators and the other of said groups consisting of theeven ones of said series of resonators, and aperiodic means coupling themembers of one of said groups in tandem.

18. A band pass filter according to claim 17, wherein each of saidresonators encompass a magnetic field and an electric field in couplingrelation with each other and said means coupling said resonators inseries are so disposed with respect to the magnetic and electric fieldsof said resonators that the energy traverses said series of resonatorsalternately on the electric and magnetic fields of said resonators, saidinput terminal means being disposed in coupling relation with theelectric field of the first of said series of resonators and said outputterminal means being disposed in coupling relation with the magneticfield of the last of said series of resonators.

19. A band pass filter according to claim 18, wherein said aperiodicmeans comprises two pairs of coaxial coupling lines having a lengthequal to an odd multiple of one-quarter wavelength, one pair of saidcoupling lines coupling the first and the third resonators while theother pair of said coupling lines couple the third and the fifthresonators, one coupling line of each pair of coupling lines havingcapacitive coupling probes and the other coupling line of each pair ofcoupling lines having inductive coupling loops for magnetic and electricfield energy coupling between the members of said one of said groups.

References Cited in the file of this patent UNITED STATES PATENTS2,177,761 Wheeler Oct. 31, 1939 2,293,384 De Cola Aug. 18, 19422,418,469 Hagstrum Apr. 8, 1947 2,419,557 Friis Apr. 29, 1947 2,566,087Lerbs Aug. 28, 1951 2,649,576 Lewis Aug. 18, 1953 OTHER REFERENCESrelied on.

